Unlike a fixed or stationary site, a mobile communication terminal, such as a communication antenna that is to be pointed at a satellite, and which is mounted on a moving vehicle, such as a truck or ship, must operate in either a static or dynamic mode. In order to acquire and point at the satellite, a mobile communication terminal requires a reference for stabilizing the orientation of the antenna line of sight (LOS), and comparing it with the LOS coordinates to the satellite. Usually, a `platform reference` device is employed to measure the orientation of the mobile platform relative to gravity (normal to the earth) and geographic north, and encoders are used to measure the antenna gimbal angles relative to the platform. Included among the various types of sensor devices that may be employed for this purpose are global positioning system (GPS) receivers, inertial rate sensors, North-seeker type devices, flux-gate magnetic compasses, and gravity-referenced inclinometers, and ring laser and fiber optic gyrocompass mechanisms. As will be discussed below, each of these conventional devices has its own set of advantages and disadvantages.
A global positioning system (GPS) receiver has an accurate heading and tilt (e.g., down to 0.3 deg RMS), works on a rapidly moving platform, is immune to magnetic effects, and is low power and ruggedized, Moreover, its cost is only moderate. Among its disadvantages are the fact that it requires two meter antenna separation for full accuracy, has some RF blockage, has a low (1 Hz) update rate (with up to once second attitude output staleness), and provides attitude outputs only.
An inertial rate sensor (e.g., piezoelectric or quartz tuning fork, fluid rotor, gas flow type) has the ability to provide real-time analog rate outputs. In addition, it is a relatively inexpensive component (less than $1K per axis), works on a moving platform, and has high bandwidth. Also, it is effectively immune to magnetic effects, and is low power and ruggedized. However, it provides only rate outputs, which suffer long term drift.
A North-seeker type device, such as a mechanical, ring-laser or fiber-optic gyro, has an accurate heading (e.g, 0.1 deg RMS), typically provides tilt outputs (e.g., 0.02 deg RMS), and is ruggedized. However, it is expensive and is not well suited for a moving platform. In addition, it is massive and consumes considerable power.
A flux-gate magnetic compass is a low power, low cost device, that works on a moving platform, and is ruggedized. Unfortunately, it is affected by ferrous materials (calibration is lost if the magnetic environment changes), and it provides a heading output only, without tilt. It also suffers from poor accuracy (.about.0.5 deg at best), and its output is magnetic north, not true north.
Gravity-referenced inclinometers have accurate absolute tilt, are immune to magnetic effects, can be low cost, are ruggedized and low power. However, they are severely disturbed by motion, and provide tilt outputs only (no heading information).
A ring laser gyrocompass (RLG) or fiber optic gyrocompass (FOG) enjoys a highly accurate heading and tilt (e.g., down to 0.02 deg RMS), a high output update rate, and works on a rapidly moving platform. In addition, it is immune to magnetic effects, can provide rate outputs, and meets all military specifications. Unfortunately, such a gyrocompass system is massive (typically weighing fifty pounds), requires substantial electrical power to operate (e.g., 75 watts) typical, and is costly to manufacture.
Even though (RLG) or fiber-optic (FOG) gyrocompasses are expensive relative to the cost of communication equipment, they do provide the greatest accuracy for either static or dynamic platforms. As a consequence, they have replaced mechanical gyro systems in production, due to greater accuracy and much greater operating life. If the platform is static during operation, inclinometers can provide accurate, cost-effective tilt measurements. However, motion disturbs inclinometer outputs severely, since they cannot distinguish lateral accelerations from the desired gravitational acceleration. For dynamic platforms, various equipment suppliers integrate inertial rate sensors with inclinometers in order to provide cost-effective real-time dynamic tilt angles. However, low-cost azimuth sensors are limited to magnetic flux gate compasses, which provide about 0.5 deg accuracy relative to magnetic north.
This uncertainty of magnetic north to true north provides a further complication. If greater north accuracy is needed, for static applications, `north seekers` are available. These commonly provide both azimuth and tilt outputs. However, they cost almost as much as a complete FOG or RLG gyrocompass, but provide limited dynamics and less north accuracy. The GPS receiver provides almost as good accuracy for a much lower cost. The north seekers also lose accuracy at high latitudes, unlike the GPS receiver.
For static applications, which typically require better than 0.5 deg north error (but not better than 0.15 deg), the GPS receiver is the most cost-effective solution. However, for dynamic platforms with great than about 0.1 deg/sec rotation, the 1 Hz update rate and up to 1 sec delay in the GPS output may lead to excessive errors.